This invention relates to a very flat optical film that is formed by the melt casting method, and relates in particular to functional films such as polarizing plate protection films, retardation films and view angle expanding films used in liquid crystal display devices and antireflection films used in plasma displays, as well as optical films that can be used for various functional films such as those used in organic EL displays and the like. This invention also relates to a method for manufacturing these films.
Liquid crystal display devices are being widely used as monitors based on the fact that space and energy conservation is improved compared to CRT display devices of the prior art. In addition, the use of liquid crystal display devices in television is becoming more widespread. These liquid crystal display devices use various optical films such as polarizing films and retardation films.
However, in the polarizing film of the polarizing plate used in the liquid crystal display device, a protective film which is an optical film formed of a cellulose ester film is layered on one or both surfaces of a polarizing element comprising a stretched polyvinyl alcohol film. The retardation film is used for expanding view angle and improving contrast and the like, and is a film to which retardation has been imparted by stretching films such as polycarbonate, cyclic polyolefin resin and cellulose ester films. These optical films which are also called optical compensation films are required to have no optical defects, uniform retardation and in particular, there must be no variation in the phase axis. As monitor and TV become larger in size and have higher definition, the quality requirements become more stringent.
The methods for manufacturing optical films are largely divided into the melt casting method and the liquid casting method. The former method is one in which a polymer is melted by applying heat and cast on a support, then hardened by cooling. Stretching is then carried out to form a film if necessary. In the latter method, the polymer is dissolved in a solvent and the resultant solution is cast on a support and the solvent is evaporated. Stretching is then carried out to form a film if necessary.
In either of these manufacturing methods, melted polymer or a polymer solution is hardened by cooling or drying on a support. In addition, after being peeled from the support, the polymer is subjected to processing such as drying or stretching while being conveyed using a plurality of conveyance rollers.
In the liquid casting method for manufacturing films, there is a problem in that there is a great load on the environment since large amounts of solvent are used. Meanwhile, since the melt casting method for manufacturing films does not use solvents increased production efficiency can be expected. The melt casting method is preferable in view of environmental protection, but the film that has been formed by melt casting has the disadvantage that thickness is uneven compared to that formed using liquid casting.
Methods that have been proposed in order to improve unevenness in thickness due to die lines on the surface of the film that is formed by melt casting using norbornene resin include employing T dies that have been subjected to surface processing at the die lip portion; performing a step of adhering the film that has been extruded from the dies to a cold drum at atmospheric pressure of 50 kPa or less; and using a rust inhibitor in the dies and the like (See Patent Document 1 for example.)
[Patent Document 1] Japanese Patent Application Laid-Open Publication No. 2005-55619
In films having cellulose resin as the main component material, in addition to properties such as optical uniformity, and few optical defects, adhesion to the polarizing plate is excellent and it has adequate vapor permeability when being adhered to the polarizing plate that has been subjected to wet stretching and thus the above methods for manufacturing polarizing plate protection films are favorable. In addition, as televisions become bigger and lighter in weight, it is important that film surface flatness is ensured even in films wider than 1500 mm and in films which are thicker than 100 μm.
The film having norbornene resin as the main component which uses the technology disclosed in Patent Document 1, to some extent allows the surface smoothness consistent with the stringent product quality requirements of today's market.
However, if the same manufacturing method is used for the norbornene resin as that used in the case of the cellulose resin, films of the same quality could not be formed. In particular, it was difficult to obtain a high degree of smoothness if the size of the film is such that the width exceeds 1500 mm and the thickness is less than 100 μm.
The reason for this is not clear, but it is presumed that this is because viscosity changes due to temperature is large for the cellulose ester resin and the melting behavior is very different from that of the norbornene polymer.